The influence of hydrodynamical winds on hot accretion disk solutions

We study the effect of a possible hydrodynamical wind on the nature of hot accretion disk solutions. We find that the advection-dominated branch, in the presence of a wind, maintains the self-similar solution for the disk structure with the temperature, θ α 1/r, optical depth, τ α r^P-1/2, and accretion rate, in α r^P. Based on global solutions, cooling due to wind energy loss and advection are found to be equally important. For a wide range of viscosity and wind parameters, the temperature is about one-tenth of the virial value and P ≈ 0.9, independent of the mass accretion rate and radius. In the context of cooling by unsaturated Comptonization of soft photons, solutions also exist in which radiative cooling, advection, and wind cooling are important. In this case, wind-regulated solutions are possible. Here, the radial dependence of the critical mass accretion rate above which solutions do not exist is unchanged from those solutions without winds. The wind/advection-dominated solutions are locally unstable to a new type of instability called "wind-driven instability," in which the presence of a wind causes the disk to be unstable to long-wavelength perturbations of the surface density. The growth rate of this instability is inversely proportional to the ratio of the radiative cooling to the gravitational energy dissipation rates, and it can grow on a timescale much longer than the viscous timescale in the disk for sufficiently small radiative cooling efficiencies.